1. Field of the Invention
The present invention relates to a multi-chip color image sensor and, more particularly, to a multi-chip color image sensor in which a plurality of semiconductor image sensors each having a plurality of light-receiving windows are arrayed, and the plurality of windows in a main scanning direction constitute one color light-receiving element.
2. Description of Related Background Art
Conventionally, an original reading unit of, e.g., a facsimile apparatus utilizes semiconductor image sensors such as CCD image sensors, bipolar image sensors, and the like, whose light-receiving windows are aligned in a line.
In general, as a system of using these sensors, a so-called multi-chip system is known. In this system, a plurality of semiconductor image sensor chips each having a length of several mm to several tens of mm are arrayed to have the same size as an original to be read, and the sensor array is used as a proximity image sensor using a one-to-one correspondence optical system, e.g., a SELFOC lens array (trade name, available from Nippon Sheet Glass Co. Ltd.)
FIG. 1 is a schematic plan view of multi-chip system semiconductor image sensors.
FIG. 2 is an enlarged view of a portion A of the multi-chip system semiconductor image sensors.
FIG. 3 is a diagram showing an optical system using the multi-chip system semiconductor image sensors.
FIGS. 1 and 3 illustrate an original 1, a circuit board 4 for mounting semiconductor image sensors, on which a desired circuit is arranged, semiconductor image sensors S.sub.1 to S.sub.n which are aligned in a line, a SELFOC lens array 5, and an original illumination LED 6.
In FIG. 2, light-receiving windows 7 of each semiconductor image sensor are aligned in a line.
The multi-chip system semiconductor image sensors have the following features. That is, they do not require a reduction optical system, and can be rendered compact. Thus, the multi-chip system semiconductor image sensors are widely used in image processing apparatuses such as facsimile apparatuses.
In recent years, a demand has arisen for color sensors in addition to the above-mentioned monochrome semiconductor image sensors. As a most general system for realizing a color system, the above-described multi-chip system semiconductor image sensors have been examined. For example, the following three systems are proposed.
In the first system, a pixel (light-receiving window) pitch is set to be 1/3 that of a monochrome image sensor, and RED, GREEN, and BLUE color filters are arranged on the light-receiving windows in turn to constitute a light-receiving unit for one line, so that outputs from three adjacent light-receiving windows, i.e., RED, GREEN, and BLUE light-receiving windows are defined as one color dot (one color light-receiving element).
FIG. 4A is an explanatory view showing an arrangement of light-receiving windows for monochrome sensors, and FIG. 4B is an explanatory view showing an arrangement of light-receiving windows for color sensors.
In FIGS. 4A and 4B, semiconductor image sensors 8a and 8b respectively have light-receiving windows 7a and 7b. A pitch (a light-receiving window pitch in the main scanning direction) of the light-receiving windows 7b of the color sensors is 1/3 a pitch (light-receiving window pitch in the main scanning direction) of the light-receiving windows 7a of the monochrome sensors. RED, GREEN, and BLUE color filters are arranged in turn on the light-receiving windows 7b. Note that such a system is called an in-line system.
In the second system, the number of lines of the light-receiving windows on the semiconductor image sensors is increased from one to three, and an interval between adjacent lines is set to be the same as or an integer multiple of the light-receiving window pitch in the main scanning direction. For example, when the interval between the adjacent lines is set to be the same as the pitch in the main scanning direction, outputs from the first and second lines are input to a memory, so that the output from the first line is delayed by two lines, and the output from the second line is delayed by one line. Thus, one color dot (one color light-receiving element) is defined by the two delayed outputs and the output from the third line.
FIG. 5 is an explanatory view showing an arrangement of light-receiving windows.
In FIG. 5, semiconductor image sensors 10 have light-receiving windows 9. The light-receiving windows 9 are separated by a pitch P in the main scanning direction, and are separated by a line pitch Q (light-receiving window pitch in the subscanning direction). For example, RED color filters are arranged on the light-receiving windows of the first line, GREEN color filters are arranged on the light-receiving windows of the second line, and BLUE color filters are arranged on the light-receiving windows of the third line.
In the third system, three lines of semiconductor image sensors each having light-receiving windows are arranged, and color filters are arranged in units of lines of semiconductor image sensors.
FIG. 6 is an explanatory view showing an arrangement of semiconductor image sensors.
In FIG. 6, semiconductor image sensors 11, 12, and 13 of the first, second, and third lines have light-receiving windows 7. For example, RED color filters are arranged on the light-receiving windows of the semiconductor image sensor of the first line, GREEN color filters are arranged on the light-receiving windows of the semiconductor image sensors of the second line, and BLUE color filters are arranged on the light-receiving windows of the semiconductor image sensors of the third line.
In the above-mentioned prior art, the subscanning direction corresponds to a direction in which an original to be read is fed, and the main scanning direction corresponds to an array direction of light-receiving windows. A "column" means an array of light-receiving units in the subscanning direction, and a "row" means an array of light-receiving windows in the main scanning direction.
FIG. 7 is an explanatory view showing the main scanning direction and the subscanning direction.
FIG. 7 illustrates a subscanning direction 16, a main scanning direction 17, a read start position 14 of a light-receiving window of the first chip of multi-chip image sensors, and a final read position 15 of the last chip.
However, the multi-chip color image sensors of the above-mentioned three systems suffer from the following problems.
In the in-line system as the first system, for example, a dot pitch at a joint of adjacent chips of the semiconductor image sensors is considerably larger than a regular pitch, as shown in FIG. 8.
In the in-line system, a dot pitch equivalently corresponds to a pitch between adjacent GREEN, RED, or BLUE windows since RED, GREEN, and BLUE light-receiving windows constitute one color dot (one color light-receiving element) in a color sensor. FIG. 8 illustrates a pitch between adjacent GREEN windows.
If a regular pitch is represented by P.sub.1 and a pitch at a joint of adjacent semiconductor image sensor is represented by P.sub.2, P.sub.2 is given by: EQU P.sub.2 =P.sub.1 +2A+B-D 1
where A is the distance between an edge of the last light-receiving window and an edge of a semiconductor image sensor chip, B is the gap at a joint between semiconductor image sensors, and D is the distance between adjacent light-receiving windows.
The dot pitch P.sub.2 (to be simply referred to as the pitch P.sub.2 hereinafter) at a joint between adjacent semiconductor image sensor chips is larger than the regular pitch P.sub.1 (to be simply referred to as the pitch P.sub.1 hereinafter) by 2A+B-D.
More specifically, the pitches P.sub.1 and P.sub.2 are calculated at a resolution of 400 DPI (400 dots per inch).
The pitch P.sub.1 is calculated by: ##EQU1##
A is set to be 20 .mu.m in consideration of the influence on light-receiving windows when semiconductor image sensors are cut from a semiconductor wafer.
B is set to be about 20 .mu.m in accordance with die-bonding precision of semiconductor image sensor. D is set to be about 6 .mu.m in consideration of isolation of light-receiving windows. In this case, the pitch P.sub.2 is calculated by substituting P.sub.1 =63.5 .mu.m, A=20 .mu.m, B=20 .mu.m, and D=6 .mu.m in equation 1 as follows: ##EQU2## Therefore, ##EQU3##
In a general color sensor, a pitch offset of about 1.85 times the regular pitch forms a nonsensitive band at a joint to cause moire, and adversely influences an output image.
The 3-line system as the second system can cope with a pitch offset easier than the in-line system, as shown in FIG. 9.
In the 3-line system, since the RED, GREEN, and BLUE color filters are arranged in units of rows, a regular pitch P.sub.3 corresponds to a pitch between adjacent color filters, and a dot pitch P.sub.4 at a joint of adjacent semiconductor image sensor chips corresponds to a pitch between color filters at end portions of adjacent semiconductor image sensor chips, as shown in FIG. 9.
Assuming that the pitch P.sub.3 is set to be 63.5 .mu.m, and a main scanning length of a light-receiving window is set to be 30 .mu.m, the dot pitch P.sub.4 is calculated by: EQU P.sub.4 =2A+B+30 (.mu.m)=90 (.mu.m)
At this time, the pitch P.sub.4 is about 1.42 times the pitch P.sub.3. This value poses no problem on an output image since a pitch offset which influences an image is 1.5 times or more the regular pitch in general. However, the 3-line system requires an external memory, and the entire system is completed, resulting in an increase in cost.
The third system requires a memory having a larger capacity than that required in the 3-line system described above, and mounting precision of semiconductor image sensors must be improved. Therefore, it is considerably difficult to realize the third method.
In order to prevent formation of the nonsensitive band in the main scanning direction, another system is proposed. In this system, as shown in FIGS. 10A and 10B, every other sensors are offset in the subscanning direction, and overlap each other in the main scanning direction.
A sensor arrangement in which a plurality of chips are alternately arranged on a circuit board 1, as shown in FIGS. 10A and 10B, is called a staggered arrangement. The staggered sensors have the following feature. That is, there is no change in resolution at a joint between adjacent chips since a pitch of color filters need not be changed for data at the joint between the adjacent chips.
However, the staggered sensors require a space d between adjacent sensors, which space is considerably wider than a pitch x between adjacent color filters. Thus, a memory having a large capacity is required to synchronize output signals from the sensors.
As described above, the above-mentioned systems for constituting the multi-chip color image sensors suffer from their own problems. The in-line system as the first system has received a lot of attention due to its advantages, e.g., a simple system, a possibility of a reduction in cost, and the like, since it does not require a memory.